Citigroup in March added restrictions on firearms sales for new retail-sector clients after a mass school shooting in Parkland, Florida, drawing the ire of the U.S. gun lobby.

"Recent news reports indicate that the Commissioner 'castigat them for straying into social policy' while 'glowering and speaking emphatically' about guns," the senators said in the letter.

"Most concerning are reports that Commissioner Piwowar delivered a 'thinly veiled threat' by suggesting that the Commission would withhold support for Citigroup’s regulatory request due to the company's unrelated and entirely lawful corporate practices relating to firearms." (Reporting by Doina Chiacu and Susan Heavey; Editing by Chizu Nomiyama)